Reactivity and d Orbital Participation

The three nitrogen atoms and the fluoro-substituted phosphorus atoms are coplanar (within 2.5 pm), but the phenyl-substituted phosphorus atom lies 20.S pm above this plane. The explanation offered is that the more electropositive phenyl groups cause an expansion of the phosphorus d orbitals, less efficient overlap with the p orbitals of the nitrogen atom, and a weakening of the it system at that point. This allows27 the ring to deform and the Ph2P moiety to bend out of the plane. 

Further examples of the jeopardy involved in casually dismissing d orbitals participation are the findings of Haddon and coworkers28 that d-orbilal participation is especially important in S4F4. which is nonplanar. and also that it accounts for about one-half of the delocalization energy in the one-dimensional conductor (SN)4. In the latter case, the low electronegativity of the d orbitals (see Chapter 5) increases the ionicity of the S—N bond and stabilizes the structure. 

There is one important aspect of the chemistry of both silicon and phosphorus which differs markedly from that of carbon. Consider the following reactions  

In contrast to the inertness of carbon halides, the halides of silicon and phosphorus are extremely reactive with water, to the extent that they must be protected from atmospheric moisture. A clue to the reactivity of these halides is provided by the somewhat similar reactivity of acid halides which readily react with water. 

The unsaturation of the carbonyl group provides the possibility of the carbon expanding its coordination shell from 3 to 4, thereby lowering the activation energy. Carbon tetrahalide cannot follow a similar path, but the halides of silicon and phosphorus can employ 3d orbitals to expand their octets  

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First- and Second-Row Anomalies 858 The Use of p Orbitals in Pi Bonding 861 The Use (or Not) of d Orbitals by Nonmetals 86ft Reactivity and d Orbital Participation 875... 

The first quantum chemical study of thiophene was due to Wheland and Pauling29 who used a model neglecting d-orbital participation and demonstrated that the inductive effect must be allowed for in order to explain its chemical reactivity. Valence-bond12(VB) (cf. ref. 29a) and free-electron (FE) treatments30 followed. A novel approach was devised by Longuet-Higgins13 who studied a model in which the... 

Type B azapentalenes behave as one ring, but aromatic character in systems such as 485 seems to be more localized in the ring that lacks the heteroatom X. The nature of X is important, since sulfur derivatives are the most stable this is probably due to a reduction of angle strain and/or to the participation of d-orbitals. Physicochemical measurements, chemical reactivity, and quantum-mechanical calculations show... 

By virtue of unoccupied d-orbitals, iron binds to many ligands - preferably to their oxygen, nitrogen, and sulfur atoms. In enzymes and other metalloproteins, iron participates in a large number of biochemical reactions. Its chemical reactivity changes due to the oxidation state, electron spin state and redox potential, the latter ranging from +1000 mV in some heme proteins to —550 mV in some bacterial ferredoxins (Cammack et al. 1990). 

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